Carwin last fought at UFC 131 in June 2011 against Junior dos Santos in a heavyweight title eliminator. He took Dos Santos the distance, but dropped a unanimous decision.

Several months later, he said he expected to be back by the start of 2012. But not long after that, he revealed back surgery would sideline him until the middle of 2012.

That recovery took longer than planned, but Carwin hoped to return in the fall. While recovering, he was tapped to coach Season 16 of "The Ultimate Fighter" opposite Roy Nelson, with a fight planned for December.

But a month before the fight, a knee injury knocked Carwin out of that bout.

And after a neck surgery in 2011, the back surgery and the knee injury, the 38-year-old Carwin elected to hang up the gloves.

Carwin signed with the UFC in 2008 after starting his pro career 8-0 with eight first-round stoppages, including Ring of Fire's heavyweight title.

His run of first-round finishes continued in the UFC with knockout wins over Christian Wellisch, Neil Wain and Gabriel Gonzaga.

In March 2010, with champion Brock Lesnar sidelined, Carwin fought Frank Mir for the interim heavyweight title and finished Mir with a "Knockout of the Night" performance â?? also in the first round.

That win set up a title unification fight against Lesnar, and through five minutes, Carwin appeared on the way to the belt. After taking a beating from Carwin, Lesnar survived the round â?? and Carwin had nothing left in the tank in the second, eventually tapping to an arm-triangle choke.

Carwin underwent his neck surgery later that year, then returned for the fight with Dos Santos â?? which would be his last.

Amazingly, in Carwin's 12-0 start to his career, his fights averaged just 81 seconds.